Protocycloceras minor, Kröger & Pohle, 2021

Kröger, Björn & Pohle, Alexander, 2021, Early-Middle Ordovician cephalopods from Ny Friesland, Spitsbergen - a pelagic fauna with Laurentian affinities, European Journal of Taxonomy 783 (1), pp. 1-102 : 56-57

Protocycloceras minor sp. nov.

urn:lsid:zoobank.org:act:C3D57CD1-2758-40A3-8596-6865D77F3620

Figs 36B–C, E–H , 37C , 38

Etymology

From the Latin ‘ minor ’, ‘relatively small’, referring to the relatively small size of this species of Protocycloceras .

Type material

Holotype Specimen FMNH-P30411 .

Paratypes Five additional specimens ( FMNH-P30410 , P30412 to P30415 ) all from type horizon and locality.

Type locality and horizon

From Profilstranda section, adjacent to Hinlopenstratet, Spitsbergen, bed PO 131, 128 m above base of Olenidsletta Member, V 2 trilobite zone, Blackhillsian, Floian.

Description

The holotype is a fragment of a body chamber and part of the phragmocone with a nearly circular conch cross section of 9.5 mm in diameter ( Fig. 36B, E–F ). The preservation does not allow precise measurement of the angle of expansion. The preserved part of the body chamber is 12 mm long. The conch is annulated, with annulations running parallel to the sutures with a distance of ca 2 mm and a chamber length of ca 2 mm (ca 4–5 chambers and annulations per corresponding cross section diameter).

The siphuncle is nearly tubular, has a diameter of 2.1 mm (rSD = 0.22), and is located ca 0.7 mm from the conch margin (rSP = 0.07) ( Fig. 37C ). The holotype has two complete septa preserved with a distance of 2.2 mm at a corresponding conch cross section of 9.5 mm (rCL = 0.23). The septal necks are poorly preserved but appear to be orthochoanitic and with a length of ca 0.5 mm.

The conch cross section is circular to subcircular and grows with an angle of expansion of ca 3° in the two other measured specimens (FMNH- P30410, FMNH-P30414). The septal perforation was measured in FMNH-P30412 and FMNH-P30415 with some inaccuracy because of poor preservation and varies from 0.29 to 0.34 in the two specimens. The conch surface in all specimens is ornamented with ribs and transverse striae. The ribs are directly transverse and run parallel to the sutures; they are very slightly shifted toward the apex at the prosiphuncular side, forming a very shallow ventral sinus. The ribs are more sharply rounded than the valleys, ca one rib and one concave interspace occur per chamber, and ca 3 ribs occur per distance similar to corresponding conch cross section. The relative spacing of the annulation increases with increasing conch diameter ( Fig. 38 ). Additionally, the surface is ornamented with ca 15–20 fine transverse striae or growth lines per annulation. The largest specimen in the collection has a diameter of ca 12 mm (specimen FMNH-P30415).

Comparison

The siphuncle is more eccentric (rSP ≈ 0.1 in P. minor sp. nov. compared with 0.25 in P. lamarcki ) and thinner (ca ⅓ of the conch cross section in P. lamarcki ) than in P. lamarcki . The position and dimensions of the septal perforation are similar to those of Castelloceras arennigense Evans, 2005 , from the Bohaul Member of the Ogof Hên Formation, Moridunian Regional Stage (Floian), which is slightly older than division V2 of the Olenidsletta Member. However, the details of the siphuncle are not known from that species, which precludes further comparison.

Figures

Fig. 36. Protocycloceratid cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Protocycloceras sp. FMNH-P30417, lateral view, from bed PO 123.3. B–C, E–H. Protocycloceras minor sp. nov. B, E–F. Specimen FMNH-P30411, holotype, from bed PO 131. B. Ventral view, prosiphuncular side. C. Specimen FMNH-P30412, from bed PO 131, with details of ornamentation well preserved. D. Catoraphiceras sp., FMNH-P30425, from bed PO 131, oblique lateral view. E. Adoral view of holotype with septal perforation visible. F. Fragment of holotype with chamber spacing G. Specimen FMNH-P30413 showing the shape of the septal curvature, dorsal view, antisiphuncular side. H. Specimen FMNH-P30410, with conch surface well preserved. I. Protocycloceras lamarcki (Billings, 1859), FMNH-P30418, from bed PO 04, with poorly preserved conch surface. Scale bars = 5 mm.

Fig. 37. Longitudinal cross sections of protocycloceratid cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A–B. Protocycloceras lamarcki (Billings, 1859). A. Specimen FMNH-P30418, approximal dorso-ventral median section with details of heavily recrystallized siphuncle and sepal necks, showing concave siphuncular segments. B. Specimen FMNH-30409, apical fragment of phragmocone with cameral deposits and siphuncle with concave segments and orthochoanitic septal necks. C. Protocycloceras minor sp. nov., FMNH-P30411, holotype, with poorly preserved details; siphuncle with nearly tubular segments and probably short orthochoanitic septal necks. Scale bar = 2 mm for all figures.

Fig. 38. Diagram of the spacing of the annulation in Protocycloceras minor sp. nov. from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. See Supp. file 1 for details of measurements.